It’ll work the same!
It’s a psychic knack, so it’s not tied to anything physical going on. If someone senses he’s there, whether they see him or not, he’s going to know. This includes a blind or normal human hearing him close by. It’s a very handy ability to have when he’s avoiding notice!
(Sorry for how long it’s taking to answer these, life has been tough this month)
It is a psychic ability, so it would respond the same as it would to anyone else seeing him. He’d get a general sense of unease, the strength of it depending if he’ll be in any danger from the person seeing him, and know to duck instantly out of sight, hopefully leaving the birdwatcher to assume they’re seeing things. Range doesn’t matter for this ability, only the act of being seen.
Brothers Apart 